Now to the debate over knives being allowed on airplanes. As we reported here, the 9/11 mastermind once claimed the terrorists claimed using those pocket knives. Tonight the tsa on the defensive over allowing small knives on planes again. The head of the agency now taking questions from congress. Abc's david kerley takes us in the room. Reporter: It's not pocket knives. This is what keeps the head of the tsa up at night. Reporter: That is a recreation of a foiled bomb plot with a small sized explosive. John pistole, who plans to allow small knives back on planes, showed video to skeptical lawmakers. If you're asking whether i think individuals with those small pocket knives could take over an aircraft, take control of it, I don't think so. Reporter: Pistole says pocket knives aren't a significant threat, but take a lot of time in those security lines. 2,000 confiscated every day, with each incident taking two to three minutes. But now even tsa officers take more time to screen. Why?E knives. All look similar. With small enough blades. But only one would be allowed -- this one would be banned because the blade locks. This one can't go on board because of its molded handle. And screeners share a worry of flight attendants. We believe that it will endanger the security of the screeners at the airports. Reporter: But pistole wants his screeners concentrating on something else. It is not pocket knives. It's not pocket knives. Reporter: What is it? Can you tell me? It is those nonmetallic improvised explosive devices. The bombs that are very small, they are concealable, and they are well designed. Reporter: Thay the tsa head displayed how little liquid or chemicals it takes. Not much bigger than this cup of water. Reporter: To do this. Reporter: The tsa boss is sticking with his plan, he is not backing down, even though some members today, said his plan is flawed. David kerley, our thanks to you. Turning now to the economy tonight and to the stock market,

This transcript has been automatically generated and may not be 100% accurate.

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